Limits of reading

NAME: DATE:
LIMITS OF READING (LOR) MEASUREMENTS
In science, the results of experiments are never 100% reliable. There are always experimental errors involved. It is therefore important to be able to assess the magnitude of these and their effect on the reliability of the final result. It is important to differentiate between the accuracy and the precision of a result. The accuracy of a measurement refers to the "closeness" of a numerical value to the actual or theoretical value. The accuracy is usually measured as the percent difference from the theoretical value using the expression:

Percent difference =

The precision refers to the “repeatability” of a measurement. The two factors that determine the precision are the measuring instrument (LOR) and the “measurer,” the person making the measurement.

READING A GRADUATED CYLINDER

The meniscus is the curve seen at the top of a liquid in response to its container. The meniscus can be either concave or convex. A concave meniscus (ex: water in glass) occurs when the molecules of the liquid are more strongly attracted to the container than to each other. A convex meniscus (ex: mercury in glass) is produced when the molecules of the liquid are more strongly attracted to each other than to the container. In some cases, the meniscus appears flat (ex: water in some plastics).

When you read a scale on the side of a container with a meniscus, such as a graduated cylinder, it's important that the measurement accounts for the meniscus. Measure so that the line you are reading is even with the center of the meniscus. For water and most liquids, this is the bottom of the meniscus. For mercury, take the measurement from the top of the meniscus. In either case, you are measuring based on the center of the meniscus.

http://www.youtube.com/watch?v=7ewRaV5baik

Equipment
Picture
Limit of Reading (LOR)
Half of LOR
Measurement
Ruler*

Beaker*

10 mL graduated cylinder*

*NOTE: For analogue equipment (with divisions), record to the limit of reading + half the LOR. For digital equipment, record all the dgits displayed.
Equipment
Picture
Limit of Reading (LOR)
Half of LOR
Measurement
100 mL graduated cylinder*

Digital scale (2 decimal places)

Temperature probe